When Ariane Sherine's brilliant initiative for an "atheist bus" campaign was in its planning stages, I wrote to her to say that I was not happy about the word "probably" in the slogan "There is probably no God".

I would question the rationality of anyone who thought that there is probably no Father Christmas, or probably no fairies at the bottom of the garden, etcetera, and since such beliefs and beliefs in the gods of Olympus and Ararat and all other religions are on a par, there is no "probably" about it. To which Sherine replied that the advertisement would not be accepted by the bus companies unless it contained the word "probably".

According to Tim Bleakley, marketing director of CBS Outdoor, which handles advertising for the bus networks, "advertising guidelines" require the word "probably"; to say that there is no God, he said, "would be misleading … So as not to fall foul of the code, you have to acknowledge that there is a grey area".

It would be misleading, eh? Thus the metaphysical authority of advertisers. You have to take your hat off to this one. If one wished to cite a better example of insidiousness, pusillanimity, timidity and absurdity, you would be hard pressed. There is something delicious about the thought of a functionary in an advertising agency doing ontology by arbitrating on the question of which fictional characters need a grey area of uncertainty around discussion of their existence – Little Red Riding Hood? Rumpelstiltskin? Santa? Betty Boop? Saint Veronica (who allegedly started out as sweat on a cloth and became a person)? Aphrodite? Wotan? Batman?

And of course the inevitable has immediately happened. Theos, the religious "think" tank, clutching at straws, claims that the word "probably" might encourage some to take the slogan to mean the opposite of what it is intended to mean. Such is the way with theology.

But give a nanometer to the ever-hopeful faithful – hope being a virtue after all – and they will take ten thousand miles, bringing with them on the journey all the things for which religion is so notable, from fantasy through folly to febrility and fanaticism.

Well: let us for a moment take the advertising standards code seriously. Parity requires that in all the many advertisements promoting religious belief on the buses and underground trains, "allegedly" be inserted into claims and statements that imply the existence of supernatural agencies. Now that the gauntlet has been thrown down on "probably" for the atheist buses, let us demand that "allegedly" appear in all advertisements promoting the opposite view. I shall be writing to Tim Bleakley (CBS Outdoor, Camden Wharf, 28 Jamestown Road, London NW1 7BY) and the Advertising Standards Authority on the subject today, and invite you all to do likewise.